Development of Focal Plane Arrays Utilizing NbN Hot Electron Bolometric Mixers for the THz Regime

Transcription

1 Development of Focal Plane Arrays Utilizing NbN Hot Electron Bolometric Mixers for the THz Regime E. Gerecht 1, C. F. Musante 2, Y. Zhuang 2, M. Ji 2, and K. S. Yngvesson 2 1 Department of Physics and Astronomy, 2 Department of Electrical and Computer Engineering, University of Massachusetts, Amherst, MA T. Goyette and J. Waldman Submillimeter Technology Laboratory, University of Massachusetts at Lowell Research Foundation, Lowell, MA AB STRACT - Im prove ments in de vice de vel op ment and quasi-op ti cal cou pling tech niques uti liz ing pla nar an ten nas have led to a sig nif i cant achieve ment in low noise submillimeter wave re ceiv ers at pro gres - sively higher fre quen cies. Hot Elec tron Bolometric (HEB) re ceiv ers made of thin film super con duct ing films such as NbN have pro duced a vi a ble op tion for in stru ments de signed to mea sure the mo lec u lar spec tra for as tro nom i cal ap pli ca tions as well as in re mote sens ing of the at mo sphere. To tal sys tem DSB re ceiver tem per a tures of 500 K at 1.56 THz and 1,100 K at 2.24 THz were mea sured since the last STT Sym po sium. These re sults are 13 and 20 times the quan tum noise limit at the re spec tive fre quency (the DSB quan tum noise limit (hf/2k) is about 24 K at 1 THz). Typ i cal best per for mance for Schottky bar rier mix ers is about 100 to 200 times the quan tum noise limit. The tech nol ogy of NbN Hot Elec tron Bolometric (HEB) mix ers is pro gress ing from the one pixel plat form into a multi pixel sys tem and spe cial con sid er ations of the new re - quire ments for such de vices is em pha sized. One im por tant char ac ter is tic is the LO power con sump tion which is in the hun dreds of nanowatts range and, there fore, makes NbN HEB mix ers ex cel lent de vices to in - te grate with a num ber of prom is ing power sources un der de vel op ment as well as avail able tech nol o gies. Fur ther more, new de vel op ments are un der way which will de crease the op ti cal and mi cro wave cou pling loss fur ther; in par tic u lar, im prove ment of the RF match of the de vice to the an tenna, op ti mi za tion of the in - put im ped ance of the IF am pli fier, and fur ther im prove ment of the NbN film ac tive me dium qual ity. Pre lim - i nary study of MgO sub strates shows an im proved IF band width. IF noise bandwidths in ex cess of 10 GHz are ex pected in the near fu ture. The re cent re sults re ported here make the de vel op ment of fo cal plane ar rays with tens of HEB mixer el - e ments on a sin gle sub strate for real time im ag ing sys tems in the THz re gion an achiev able goal. I. INTRODUCTION The de vel op ment of low-noise re ceiv ers in the THz fre quency re gion is pri mar ily mo ti vated by the need for low noise and low power con sump tion re ceiv ers for the next gen er a tion of space-based and air - borne as tro nom i cal ob ser va to ries (FIRST, SO FIA, etc.), as well as space-based re mote sens ing of the Earth s at mo sphere (EOS-MLS). Un til a few years ago, the only het ero dyne re ceiv ers avail able for the THz

2 re gion uti lized non lin ear fre quency-con ver sion de vices which were ei ther GaAs Schottky Bar rier Di odes (SBD) or InSb Hot Elec tron Bolometers (HEB). THz SBD mixer tech nol ogy has re cently made a tran si tion from cum ber some whis kered di odes in cor ner-cube mounts to pla nar ver sions in wave guide. The Dou ble Side Band (DSB) re ceiver noise tem per a ture of SBD mixer re ceiv ers has re mained es sen tially sta tion ary at about ( )x hf/2k [1] (hf/2k is the quan tum limit for DSB re ceiver noise tem per a ture and is about 24 K at 1 THz). Fab ri ca tion tech nol ogy and ma te rial pa ram e ters limit the size of the mono lithic junc tion and there fore limit the noise tem per a ture per for mance. In ad di tion, SBD re ceiv ers re quire a few mw of LO power. InSb mix ers have al ways been too re stricted in band width (only about 1 MHz) for most ap pli ca tions. Be low 1 THz, SIS (Su per con duc tor/in su la tor/su per con duc tor) mixer re ceiv ers have ex cel lent noise tem - per a ture (only a few times the quan tum noise limit). The noise per for mance is lim ited to fre quen cies be low or about equal to the super con duct ing bandgap fre quency. Hot Elec tron Bolometric (HEB) mix ers, which use non lin ear heat ing ef fects in su per con duc tors near their tran si tion tem per a ture, have be come an ex cel lent al ter na tive for ap pli ca tions r e quir ing low noise tem - per a tures at fre quen cies from 1 THz up to the Near IR. There are two types of super con duct ing HEB de - vices, the Phonon-Cooled (PC) ver sion [2], and the Dif fu sion-cooled (DC) ver sion [3][4]. At pres ent, most of the low est re corded re ceiver noise tem per a tures have been ob tained with the PC type HEB [5][6], al - though the dif fer ence is not very large. This pa per only de scribes the de vel op ment of the PC HEB. Super - con duct ing HEB mix ers also re quire much less LO power than SBD re ceiv ers (100 nw to 1 µw for PC HEBs). The only prac ti cal LO source, pres ently avail able, is an FIR gas la ser al though solid state LO sources with suf fi cient amount of power are un der de vel op ment and will be avail able in the fu ture. The pres - ent state-of-the-art of dif fer ent THz re ceiv ers is com pared in FIG. 1. FIG. 1. Noise tem per a tures as a func tion of fre quency for re ceiv ers in the terahertz re gime.

3 The con ver sion gain and out put noise of an HEB mixer can be cal cu lated us ing what has be come the stan dard model for HEB de vices [7][8]. It is found that there is an op ti mum amount of LO power which yields the min i mum noise tem per a ture. In prac tice the op ti mum re ceiver noise tem per a ture oc curs for a bias cur rent which is about % of the cur rent in the re sis tive re gion of the I-V curve with out LO power. The stan dard model, which as sumes a uni form elec tron tem per a ture, is use ful for a first or der de - scrip tion of the PC HEB, but can not give a com plete de scrip tion of the de vice. New mod els for the mixer op er a tion for fre quen cies in the terahertz re gime as sum ing non-uni form elec tron tem per a ture are un der in - ves ti ga tion [9]. The IF band width for the con ver sion gain is de ter mined by the ther mal time-con stant (τ m ) of the HEB de vice. The HEB dis si pates the power it ab sorbs through a two-stage pro cess: the heated elec trons first emit phon ons, which will then be trans mit ted through the film/sub strate in ter face into the sub strate. An in ter face re sis tance due to phonon mis match has to be taken into ac count, and this re sis tance var ies with the sub strate upon which the thin film is de pos ited. To max i mize the IF band width, the film should be as thin as pos si ble while still hav ing good super con duct ing prop er ties (high T c and low T c ). The mixer time-con stant (τ m ) also in cludes a fac tor which de pends on the self-heat ing of the bolometer [7]. The re ceiver noise tem per a - ture band width (B NT ) is wider than the con ver sion gain band width (B G ). The fact that the re ceiver noise tem per a ture band width is two to three times wider than the con ver sion gain band width is a well-known fea - ture of HEB mix ers. This char ac ter is tic can be un der stood if one re al izes that the main noise pro cess in the de vice (tem per a ture fluc tu a tion noise) yields a noise out put which falls at the same rate as the con ver sion gain, flat ten ing the net re ceiver noise de pend ence on the IF fre quency. II. DEVICE DESIGN AND FABRICATION A typ i cal HEB de vice is made from a thin (3 to 4 nm) film of NbN de pos ited on a sub strate of sil i con, quartz, sap phire or MgO by DC mag ne tron sput ter ing. Thin ner films are de sir able in or der to achieve wider IF band width. The crit i cal tem per a ture of the NbN film is about 10 K, de pend ing on film qual ity and thick - ness, and ef fi cient mix ing oc curs at about half that tem per a ture. Much ef fort has been spent on im prov ing the qual ity of the NbN films, which is es pe cially crit i cal for the thin nest films. Above the super con duct ing bandgap fre quency (roughly 1 THz for these films), terahertz ra di a tion sees a re sis tance roughly equal to the nor mal re sis tance, which is 300 Ω/square to 600 Ω/square. A de vice with an as pect ra tio (length to width) of from 1:5 to 1:10 will there fore match a typ i cal an tenna im ped ance of 75 Ω. The crit i cal cur rent of a de vice is a few hun dred µa, while a typ i cal DC bias volt age is 1 mv. Since the de vice acts as a bolometer, the ab sorbed LO power, which is a func tion of the de vice area, is mea sured by the de vice it self and is com - puted from its I-V curve. Our de vices have a length of 0.6 to 1 µm and LO power from 0.5 to 1 µw.

4 Quasi-op ti cal cou pling is very con ve nient at the very high THz fre quen cies where waveguides be come in creas ingly dif fi cult to man u fac ture. We cou ple our de vices through a 4 mm di am e ter el lip ti cal lens made from high-pu rity sil i con. In or der to fa cil i tate test ing over a wide range of fre quen cies, we have ini tially used a log pe ri odic self-com ple men tary toothed an tenna. This de sign is scaled from the mil li me ter wave de - sign in [10] and is il lus trated in FIG.2. Other antennas un der in ves ti ga tion in a num ber of lab o ra to ries are spi ral an ten nas, twin di pole/slot an ten nas, and slot ring an ten nas. We have used a log-pe ri odic an tenna with a max i mum fre quency of 3.4 THz most re cently (des ig nated as An tenna C). Our log-pe ri odic an ten nas have a 4:1 band width. The an tenna is fab ri cated from a gold film us ing lift-off li thog ra phy. At the mo ment, we use no re flec tion match ing for the sil i con lens (e r = 11.8). Op ti cal losses should de crease by about 2 db once a suit able ma te rial for such coat ings in the THz range. One such ma te rial, which is un der in ves ti ga - tion, is parylene [11][12]. The HEB re ceiver is cooled in an IRLAB liq uid he lium dewar, and THz ra di a tion en ters the dewar through a 0.75 mm thick poly eth yl ene win dow, as shown in FIG.3. The mixer is con nected through a bias tee and a semi-rigid co ax ial ca ble to a cooled HEMT IF am pli fier. In the most re cent ex per i ments, the IF chain noise tem per a ture was es ti mated to be 7 K with a band width from 1250 MHz to 1750 MHz. The LO source was a difluoromethane gas la ser, which could be made to lase ei ther at 191 µm wave - length (1.56 THz) or at 134 µm (2.24 THz) by choos ing one of two or thogo nal po lar iza tions. It has an in - var-supported struc ture which was de signed with ther mal com pen sa tion to main tain con stant cav ity length. In or der to ob tain high power sin gle mode out put, uni form cou plers con sist ing of wire grids de pos ited on a sil i con sub strate (also coated for high re flec tivity from 9-11 µm) were used. The la ser beam was mea sured to have a Gaussi an spa tial out put pro file with the first sidelobes 20 db down. The FIR la ser was pumped by an ex tremely sta ble two me ter long grat ing-tuned CO 2 -la ser. The avail able power from the CO 2 -la ser was FIG. 2. Log-pe ri odic toothed an tenna fab ri cated on a sil i con sub strate and at tached to a sil i con lens..

5 FIG. 3. Mea sure ment setup for noise tem per a ture. about 200 W [13]. A 6 µm thick my lar win dow was used as beam split ter. A di elec tric lens was used to fo - cus the la ser LO. The mw out put power of the la ser was at ten u ated by crossed wire grid polarizers in or der to set the op ti mum LO level. Al though me chan i cal chop ping of the hot/cold source is pos si ble and some times used, a typ i cal mea sure ment was per formed by in sert ing a room tem per a ture ab sorber in front of the LN 2 load by hand. The IF out put power was de tected on a power me ter and re corded in a com puter with the help of a Labview pro gram. The fact that it is pos si ble to per form the Y-factor mea sure ment with out the use of a ro tat ing chop per is a trib ute to the ex cel lent am pli tude sta bil ity of the UMass/Lowell la ser used for this ex per i ment. The am pli tude sta bil ity of the 1.56 THz la ser source, mea sured over a pe riod of min utes, with a rel a tively fast (0.1 s) in te gra tion time, was 0.3%. The sta bil ity was also ev i dent in the I-V curves re - corded by our fast (about 50 ms) com put er ized re cord ing sys tem. III. EXPERIMENTAL RESULTS FOR SINGLE-ELEMENT RECEIVERS TA BLE I gives a sum mary of data mea sured for de vices which reached DSB noise tem per a tures at 1.56 THz of 500 K and at 2.24 THz as low as 1,100 K. The out put noise tem per a ture was mea sured by com - par ing the to tal out put noise power in the op ti mum op er at ing point (with LO ap plied) with that of the de vice in the super con duct ing state (the bias volt age was de creased to zero). Since the IF noise tem per a ture was

6 TABLE I. SUMMARY OF NOISE DATA f [THz] Dev.# / Tout [K] TDSB [K] TDSB,i [K] Lc,tot [db] Lopt [db] Lc,i [db] Ant /C /C 110 1, known, we could find the out put noise tem per a ture (T out ) from this mea sure ment. The op ti cal cou pling loss was es ti mated from known losses in win dows, lens re flec tion loss, etc. The re main ing con ver sion loss is the in trin sic con ver sion loss, L c,i, which can be cal cu lated from the ory ac cord ing to the stan dard model. A set of con sis tent val ues of L c,i, T out, and T R,DSB can then be ob tained [8]. We have iden ti fied part of the in - crease (0.5 db) in op ti cal losses from 1.56 THz to 2.24 THz as be ing due to a res o nance in the poly eth yl ene win dow ma te rial. Also, the at mo spheric at ten u a tion is higher at 2.24 THz than at 1.56 THz. The ther mal noise power from the cold source had a path length of about 0.6 m be fore it reached the dewar win dow and the es ti mated at ten u a tion over this path at 2.24 THz is db. There is still an un ex plained in crease of about 2 db. Some of the in creases in op ti cal losses are in ev i ta ble but care ful op ti cal de sign should be able to elim i nate a part of this in crease with fre quency. IV. FOCAL PLANE ARRAYS WITH INTEGRATED HEB RECEIVERS In or der to fully uti lize the fu ture space-borne and air borne fa cil i ties, it will be ad van ta geous to de - velop Fo cal Plane Ar rays (FPAs) which in cor po rate the new low-noise HEB re ceiv ers. In as tro nom i cal THz ob ser va tions, for ex am ple, one of ten wants to map an area such as an in ter stel lar cloud or a gal axy. The speed with which this map ping can be done will in crease in pro por tion to the num ber of el e ments in the ar ray. Such sys tems ex ist at mil li me ter waves in ground-based tele scopes [14][15]. There are well-known lim i ta tions for the small est beam spac ings which can be ob tained [16]. These can be dis cussed in terms of the geo met ric spac ing ( x) of ad ja cent el e ments in the ar ray. If each el e ment in the ar ray il lu mi nates a tele - scope at an f-num ber of f/d, then ideal sam pling of the fo cal plane im age at the Nyquist rate re quires that x = 0.5 x (fλ/d) [15][17]. There is no type of feed el e ment which is ca pa ble of be ing spaced this close while still il lu mi nat ing the tele scope ef fi ciently [15][18]. About the best which has been achieved in prac tice is x = 1x(fλ/D), and cor ru gated horns, for ex am ple, which are very ef fi cient feed an ten nas, must be spaced at about 2x(fλ/D) [15]. The dis place ment (N) of the tele scope beam on the sky, mea sured in Full Width Half Max i mum Power (FWHM) beam widths is also re lated to x by N x/1.2λ(f/d) [16]. An ar ray el e ment spac ing of about 1.2x(fλ/D) thus cor re sponds to a spac ing of ad ja cent beams on the sky of one FWHM beamwidh.

7 There are two dif fer ent meth ods for cou pling di elec tric lenses to an an tenna ar ray: (i) a sin gle-lens con fig u ra tion; and (ii) a multi-lens con fig u ra tion. If we first con sider the sin gle-lens case, the in di vid ual el e ments placed near the fo cus of the lens will ra di ate a beam which has an f-num ber of roughly 1.0, i.e. a 56 de gree FWHM beam width. Filipovic et al. [19] an a - lyzed this case and de rived the min i mum spac ing pos si ble. To ob tain a rough es ti mate, we as sume a spac ing cor re spond ing to one beam width, and find x λ0 ε, or 35 µm for λ r 0 = 119 µm. This leads to very tight con straints on any wir ing which has to be con nected to the de vices and an ten nas, and it is ob vi ously im pos si - ble to place the IF am pli fi ers close to the an ten nas. The multi-lens con fig u ra tion, on the other hand, is much more flex i ble. The rel a tively small ( ra dius R 10 λ 0 ) el lip ti cal lens which we have de vel oped, lends it self well for use in this fly s eye type of ar ray, see FIG. 4. Both the LO and the in com ing sig nal are in jected through a quasi-op ti cal di plex er. The op tics thus are un changed from our sin gle-el e ment ap proach. The beam width from each lens is ap prox i mately given by 1.2 x λ/(2r), and the lenses can be placed at a spac ing equal to their di am e ter (2R), i.e. x=2r. The f-num - ber of the ar ray el e ments will be ap prox i mately 2R/λ ( 20), which may be about right for a typ i cal Cassegrain tele scope, with out re course to fur ther fo cus ing. The beam-scan (N) will be about one FWHM beam width. The an gu lar res o lu tion (an gu lar spac ing be tween ad ja cent pix els) will thus be about equal to the dif frac tion-lim ited beam width of the tele scope, which is typ i cal of the best res o lu tion ob tain able for FPA re ceiv ers as dis cussed above. FIG 4. A por tion of an HEB THz fo cal plane ar ray.

8 The FPA can not use the log-pe ri odic toothed an ten nas which we have em ployed so far since these are un-nec es sar ily large. The fo cal plane ar ray sys tem is also not likely to re quire the very wide band width of these an ten nas. We in stead pro pose a slot-ring an tenna as shown in FIG. 5 (a dou ble-slot an tenna would also be pos si ble). The slot-ring an tenna has been dem on strated in a four-el e ment ar ray for a 35 GHz monopulse radar [20] and also, for ex am ple, in 94 GHz MMIC receivers [21]. This an tenna is lin early po lar ized and can re ceive ra di a tion in ei ther of two per pen dic u lar po lar iza tions. There are sev eral pos si ble con fig u ra tions to ex - plore. FIG. 5 shows one such con fig u ra tion in which the LO and RF are in jected in the same po lar iza tion as in our pres ent sin gle lens re ceiver. The IF is ex tracted through a coplanar wave guide (CPW) from the point on the ring at which the THz fields have a null. It is im por tant to use air bridges in or der to can cel the even mode on the CPW. In the above-men tioned monopulse ra dar pro ject, the LO and sig nal were in jected in op po site po lar iza tions through a sim ple wire grid and two (re versed) Schottky bar rier mixer di odes were placed at the 45 de gree po si tions across the ring thus form ing a bal anced mixer. HEB de vices can not be re versed, as can Schottky di odes, but one or two de vices could be placed at the 45 de gree po si tions and this would al low very ef fi cient LO in jec tion (ide ally with out any loss) through a wire grid. The sig nal would also be in jected with - out loss, ide ally. The RF im ped ance of the HEB de vice(s) would be ad justed in the usual way by vary ing its (their) as pect ra tio for op ti mum cou pling to the ring. Dif fer ent types of fil ters can be tried on the IF line in or - der to pre vent leak age of the RF and LO through the CPW. FIG. 5 and FIG. 6 show dif fer ent ver sions of this. The en tire sil i con chip with an ten nas and NbN mixer de vices would be fab ri cated in one pro cess. MMIC HEMT am pli fier chips (size about 1 mm 2 ) would be in te grated with the mix ers by in sert ing them in etched wells in the sil i con sub strate, and trans mis sion lines could be routed on a thin layer of spun-on di elec tric. FIG. 7 shows a wide band MMIC am pli fier un der de vel op ment in col lab o ra tion with Chalmers Uni ver sity of Tech - nol ogy [cour tesy of Her bert Zirath]. The am pli fier will in clude (on chip) the ap pro pri ate im ped ance trans for - ma tion as well as bias cir cuitry for the HEB de vices. A nom i nal band width of 4-8 GHz will be suit able for FIG 5. HEB de vice cou pled to a slot ring an tenna with coplanar wave guide out put for the IF. FIG 6. A dif fer ent ver sion of the slot ring an - tenna/heb de vice.

9 many an tic i pated sys tem ap pli ca tions. An other im por tant con sid er ation is to min i mize the DC power con - sump tion of the MMIC am pli fier. ACKNOWLEDGEMENTS This work was sup ported by the Na tional Sci ence Foun da tion (ECS ) and NASA (NASA NAG5-7651). We would like to ac knowl edge Matthias Kroug of Chalmers Uni ver sity of Tech nol ogy for fab ri cat ing one of the de vices used for this re search, and the slot ring cou pled HEB de vice shown in FIG. 6. We would also like to ac knowl edge Her bert Zirath and the group of Pro fes sor Erik Kollberg at Chalmers Uni ver sity for col lab o ra tion on de sign ing IF am pli fi ers suit able for HEB fo cal plane ar rays. REFERENCES 1. P.H. Siegel, R.P. Smith, M. Gaidis, S. Mar tin, J. Podosek, and U. Zim mer mann, 2.5 THz GaAs mono lithic mem brane-di ode mixer, Proc.Ninth In tern. Space THz Technol. Symp., JPL, March 1998, p E. M. Gershenzon, G. N. Gol tsman, I. G. Gogidze, Y. P. Gousev, A. I. Elant ev, B. S. Karasik, and A.D. Semenov, Mil li me ter and Submillimeter Range Mixer Based on Elec tronic Heating of Super con duct ing Films in the Re sis tive State, So viet Phys ics: Su per con duc tiv ity, 3, 1582, D.E. Prober, Super con duct ing terahertz mixer us ing a tran si tion-edge microbolometer, Appl.Phys.Lett., 62, 2119, R.Wyss, B.Karasik, W.R. McGrath, B. Bum ble, and H. LeDuc, Noise and band width mea sure ments of dif fu - sion-cooled Nb HEB mix ers at fre quen cies above the superconductive en ergy gap, Proc. Tenth In tern. Space THz Technol. Symp., U.Vir ginia, March 1999, p P. Yagoubov, M. Kroug, H. Mer kel, E. Kollberg, J, Schu bert, H.-W. Huebers, S. Svechnikov, B. Voronov, G. Gol tsman, and Z. Wang, Hot Elec tron Bolometric Mixers Based on NbN Films De posited on MgO Sub - FIG 7. Wideband (2-12 GHz) MMIC PHEMT am pli fier de signed at Chalmers Uni ver sity. Mea sured data for gain and re flec tion co ef fi cient are also shown.

10 strates, Fourth Eu ro pean Con fer ence on Ap plied Su per con duc tiv ity (EUCAS 99), Sitges, Bar ce lona, Spain, Sep tem ber E. Gerecht, C.F. Musante, Y. Zhuang, M. Ji, K.S. Yngvesson, T. Goyette, J. Waldman, P.A. Yagoubov, M. Kroug, H. Mer kel, E.L. Kollberg, J. Schu bert, H.-W. Huebers, G.N. Gol tsman, B.M. Voronov, and E.M. Gershenzon, Im proved Noise Tem per a ture and Band width of Phonon-Cooled NbN Hot Elec tron Bolometric Mixers, Proc. In tern. Semicond. De vice Re search Symp., Char lottes ville, VA., Dec H. Ekstroem, E.L. Kollberg, B. Karasik, and K.S. Yngvesson, Con ver sion gain and noise of ni o bium hot-electron mix ers, IEEE Trans. Microw. The ory Tech niques, MTT-43, 938, K.S. Yngvesson, and E.L. Kollberg, Op ti mum re ceiver noise tem per a ture for NbN HEB mix ers ac cord ing to the stan dard model, Proc. Tenth In tern. Space THz Technol. Symp., U.Vir ginia, March 1999, p P. Khosropanah, H. Mer kel, S. Yngvesson, A. Adam, S. Cherednichenko, and E. Kollberg., A Dis trib uted Model for Phonon-Cooled HEB Mixers, Pre dicting IV Char ac ter is tics, Gain, Noise and IF band width, this con - fer ence. 10. B.K. et al., A pla nar wideband GHz re ceiver, IEEE Trans. Microw. The ory Tech niques, MTT-41, 1730, M. Ji, C. Musante, S. Yngvesson, A. J. Gatesman, J. Waldman, Study of Parylene as Anti-reflection coat ing for Sil i con Op tics at THz Fre quencies, sub mit ted to IEEE Mi cro and Guid. Lett. 12. H.-W. Huebers, priv. comm. 13. Man u fac tured by Pa cific Re search, Gorham Ave., Los An geles, CA. 14. N.R. Erickson, P.F. Gold smith, G. Novak, R.M. Grosslein, P.J. Viscuso, R.B. Erickson, and C.R. Predmore, A 15 El e ment Fo cal Plane Ar ray for 100 GHz, IEEE Trans. Microw. The ory Tech niques, MTT-40, 1 (1992). 15. P.F. Gold smith, C.-T. Hsieh, G.R. Huguenin, J. Kapitzky, and E.L. Moore, Fo cal Plane Im aging Sys tems for Mil li me ter Wave lengths, IEEE Trans. Microw. The ory Tech niques, MTT-41 (1993). 16. K.S. Yngvesson, Near-Mil li me ter Im aging with In te grated Pla nar Re cep tors: Gen eral Re quire ments and Con - straints, in In fra red and Mil li me ter Waves (K. J. But ton, Ed.), Ac a demic Press, Or lando, FL.,Vol. 10, Chap. 2, p.91 (1983). 17. D.B. Rutledge, D.P. Neikirk, and D.P. Kasilingam, In te grated-cir cuit An tennas, in In fra red and Mil li me ter Waves (K. J. But ton, Ed.), Ac a demic Press, Or lando, FL.,Vol. 10, Chap. 2, p.1 (1983). 18. K.S. Yngvesson, J.F. Johansson, Y. Rahmat-Samii, and Y.S. Kim, Re al iz able Feed-Element Pat terns and Op - ti mum Ap er ture Ef fi ciency in Multi-Beam An tenna Sys tems, IEEE Trans. An tennas and Propagat., AP-36, 1637 (1988). 19. D.F. Filipovic and G.M. Rebeiz, Off-axis Im aging Prop erties of Sub strate Lens An tennas, Proc. 5 th In tern. Symp. Space THz Tech nol ogy, May S. K. Masarweh, T.N. Sherer, K. S. Yngvesson, R. L Gingras, C. Drubin, A.G. Cardiasmenos, and J. Wolverton, Modeling of a Mono lithic Slot Ring Quasi-Optical Mixer, IEEE Trans. Microw. The ory Tech - niques, MTT-42, 1602 (1994). 21. S. Raman and G.M. Rebeiz, Sin gle- and Dual-Polarized Mil li me ter-wave Slot-Ring An tennas, IEEE Trans. An tennas and Propagat., AP-44, 1438 (1996).